Anthelmintic resistance in Australian sheep nematode populations

SUMMARY The resistance status of gastro–intestinal nematodes to anthelmintics was evaluated on 881 sheep farms throughout Australia during 1991–92. Resistance was shown to be widespread. Overall, 85% of farms had sheep infected with nematodes resistant to benzimidazole, 65% to levamisole and 34% to combination (benzimidazole + levamisole) products. Resistance to ivermectin was not detcted. On only 9% of farms did all anthelmintic groups reduce egg counts by greater than or equal to 95%. The culture of faeces from untreated sheep showed Telodorsagia circumcincta, Trichostrongylus spp, Chabertia ovina and Haemonchus contortus to be the principal species. The nature and prevalence of resistance was not significantly correlated with stocking rate. However, resistance to combination products was almost twice as prevalent on farms in areas with an average annual rainfall of greater than 500 mm.     

Anthelmintic resistance amongst sheep and goats in Kenya

Faecal egg count reduction tests (FECRT) and larval cultures were used to assess the prevalence of anthelmintic resistance amongst sheep and goats on farms served by Kenya's five regional Veterinary Investigation Laboratories. Twenty-four out of 42 farms tested (57%) showed resistance to at least one anthelmintic group. Resistance to levamisole was found in nine out of 35 sheep farms tested (26%) and 12 out of 24 goat farms (50%). Resistance to benzimidazole drugs was found in 10 out of 28 sheep farms (36%) and six out of 20 goat farms (30%). Larval cultures of post-treatment faecal samples showed resistance to be predominantly due to Haemonchus contortus. Twelve farms were routinely using substandard products which appeared to have no anthelmintic activity.     

Anthelmintic resistance in sheep in Europe: A selected review

Intensive use of anthelmintics to control gastrointestinal nematodes selects for anthelmintic resistance, which has become an important issue in many European countries. Presence of nematode strains resistant to benzimidazoles, imidazothiazoles and/or macrocyclic lactones has been repeatedly reported, particularly for the three most important genera, Haemonchus, Teladorsagia and Trichostrongylus. Additionally, multiple drug-resistant populations of these parasites have also been detected. Examples are given for the situation in European countries with differing climatic conditions and management systems of small ruminants. The widespread emergence of multi-resistant nematodes proves that the past intensive suppressive chemical control strategies may not be a successful approach any longer. Experience from the up to now development of anthelmintic resistance suggests that modern control schemes should not rely on sole use of anthelmintics, but employ other, more complex and sustainable recipes, combing chemical, environmental and immunological control.     

Anthelmintic resistance in sheep and goat farms on Peninsular Malaysia.

The faecal egg count reduction test (FECRT) was conducted on 39 sheep farms and 9 goat farms located in Peninsular Malaysia. The anthelmintic groups used in these tests were the benzimidazoles, levamisole, the benzimidazole/levamisole combination, macrocyclic lactones and closantel. Results indicated that the prevalence of resistance to the benzimidazole group was high, with approximately 50% of the sheep farms and 75% of the goat farms having resistant nematode parasite populations present. Resistance to levamisole, closantel and ivermectin was also detected. Differentiation of the infective larvae derived from faecal cultures indicated that by far the most predominant parasite species was Haemonchus contortus.     

A comparison of nematode control programs for cattle in south western Victoria

Nematodiasis and its subsequent effect on production in Hereford weaner steers in western Victoria was studied during 1983 and 1984. In the first summer, steers were allocated to 2 replicates of 6 treatments--No treatment (Nil); Morantel slow release bolus in March (M1); Morantel bolus in March and June (M2); pour-on levamisole in January, May and July (R3); albendazole in January and July (V2) and albendazole in January, May and July, (V3). In 1984, treatment M2 was discontinued to provide extra replicates for Nil and M1. The replicate paddocks were 5 ha and were stocked with 7 and 8 steers in 1983 and 1984, respectively. Nematode egg counts in faeces, were generally less than 50 epg, indicating low numbers of adult nematodes. Faecal egg counts were highest in autumn and declined during the year. There was a significant (P = 0.02) effect of treatment on mean faecal egg count. Mean egg counts for treatment groups Nil and M1 were 16 and 10 epg above the overall mean (47 epg); those of the other treatments were 6 to 12 epg below the mean. There were no significant (P = 0.8) differences between treatments in the numbers of nematode larvae on pasture, during the experiment. At the end of both years of the experiment most nematodes (92%) were early fourth stage larvae of O. ostertagi. There were no consistent differences in nematode counts between treatments. There were no significant (P = 0.33) differences between treatments in bodyweights at any time during the experiment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Anthelmintic resistance in cattle nematode in the western Pampeana Region of Argentina

During the past three years there have been several reports of cattle anthelmintic resistance (AR) in Argentina. Twenty-five cattle herds in the west fattening area of the Pampeana Region were surveyed to assess the prevalence of AR and the species of nematode involved. The survey involved fattening and cow-calf systems with a mean of 6760 bovines. Resistance was determined in vivo by a faecal egg count reduction test (FECRT). Efficacies from the World Association for the Advancement of Veterinary Parasitology (WAAVP) formula were compared with those obtained from the Abbott formula. Sixty 6-12-month-old calves were selected from each property and divided into four groups, namely, IVM (ivermectin, 0.2mg/kg), BZD (fenbendazole, 5mg/kg or albendazole 7.5mg/kg), LVM (levamisole, 7.5mg/kg) and Control (untreated group). Nematode larvae were identified by four pooled faecal cultures for each group. Cooperia and Ostertagia were the predominant nematode larvae in pre-treatment cultures. The mean FECRT efficacies for IVM, BZD and LVM were 81.0+/-21%, 93.3+/-8% and 97.6+/-4%, respectively. AR was detected in 16 (64%) herds. IVM resistance was present in 15 herds and BZD resistance in eight herds (32%). Resistance to both ivermectin and fenbendazole was present in seven herds. LVM resistance was not detected. Cooperia was the predominant post-treatment nematode in all the herds with IVM resistance. In one herd with IVM resistance, Ostertagia counts were slightly increased over pretreatment counts. Ostertagia was the prevalent post-treatment genera in herds with BZD resistance. A high level of resistance to ivermectin was recorded in this fattening region of Argentina. Also several herds with resistance to benzimidazole were observed. These results show the importance of AR in Argentinian cattle production. High correlation between WAAVP and Abbott formulae was observed. However, under some uncertain diagnosis, both formulae could be applied as complementary to help result interpretations.     

Survey of anthelmintic resistance in Western Australian sheep flocks 2. Relationship with sheep management and parasite control practices

Owners of 116 farms, whose flocks had been tested for anthelmintic resistance, were interviewed to determine their use of various sheep management and parasite control practices and their knowledge and adoption of recommended procedures for the prevention and control of resistance. Farmers knowledge of current recommendations related mainly to changing drenches and drench groups. Other aspects of the recommended program including reduction of drenching frequency and the use of alternative management strategies were not considered as important by farmers. For most questions a high proportion of farmers (greater than 20%) had no opinion. Associations between various strategies for nematode control and resistance of Trichostrongylus and Teladorsagia to thiabendazole and levamisole were examined. These relationships differed between anthelmintics and nematode genera. A number of factors were related to resistance of one or both nematode genera to one or both anthelmintic groups. These factors included flock size, percentage of ewes in the flock, cattle number, main sheep production activity, grazing strategy, frequency of drenching, changes in the frequency of drenching, number of summer drenches and the method of estimating dose rates. It was concluded that the methods employed to control anthelmintic resistance may vary with the nematode, its resistance status and the anthelmintic to which it is exposed. Modifications to the previously recommended program have been proposed which incorporate selection of the anthelmintic to be used following a test for anthelmintic resistance.     

Farm management practices associated with macrocyclic lactone resistance on sheep farms in New Zealand

AIM: To identify farm practices associated with the presence of resistance to a macrocyclic lactone (ML) anthelmintic on sheep farms in New Zealand. METHODS: A cross-sectional study was conducted to test for associations between the presence of resistance to an ML anthelmintic (ivermectin) and management practices on sheep farms in New Zealand. Selection of farms was both random (n=80) and purposive (n=32; being farms with a history of suspected ML resistance). Resistance was inferred from faecal nematode egg count (FEC) reduction (FECR) tests (FECRTs) when there was <95% reduction in FEC 7-10 days after treatment with a half dose of ivermectin (0.1 mg/kg). A logistic regression model was built to identify farm-level factors that were associated with the presence or absence of ML resistance. RESULTS: Of the 112 flock managers that were approached for interview, 103 (92%) returned useable questionnaires. The odds of ML resistance were increased: on farms that had used long-acting ML products in ewes as a pre-lambing treatment for > or =3 of the previous 5 years (odds ratio (OR) = 7.2; 95% confidence interval (CI) = 1.7-30.3); on farms where <70% of the total stock units mid-winter were from sheep (OR=6.5; 95% CI=1.6-25.6); on farms which over the year purchased >10% of the number of sheep present mid-winter (OR=7.1; 95% CI=1.5-34.7); and on farms where the average wool diameter of the main flock was <37 (OR=4.1; 95% CI=1.1-14.7) microns. The model provided a good fit to the data (pseudo R2=0.64; Hosmer-Lemeshow statistic = 0.38). CONCLUSIONS: Explanatory factors identified as associated with the presence of ML (ivermectin) resistance on farms included the use of long-acting anthelmintic formulations in ewes pre-lambing, sources of refugia of unselected parasites on the farm, breed of sheep and their requirements for anthelmintic treatments, and the importing of resistant parasites with purchased stock. The study provides support for controls that aim to provide refugia of susceptible worms and that minimise the risk of introduction of resistance through effective quarantine drenching.     

Farm management practices associated with macrocyclic lactone resistance on sheep farms in New Zealand

AIM: To identify farm practices associated with the presence of resistance to a macrocyclic lactone (ML) anthelmintic on sheep farms in New Zealand.

METHODS: A cross-sectional study was conducted to test for associations between the presence of resistance to an ML anthelmintic (ivermectin) and management practices on sheep farms in New Zealand. Selection of farms was both random (n=80) and purposive (n=32; being farms with a history of suspected ML resistance). Resistance was inferred from faecal nematode egg count (FEC) reduction (FECR) tests (FECRTs) when there was <95% reduction in FEC 7–10 days after treatment with a half dose of ivermectin (0.1 mg/kg). A logistic regression model was built to identify farm-level factors that were associated with the presence or absence of ML resistance.

RESULTS: Of the 112 flock managers that were approached for interview, 103 (92%) returned useable questionnaires. The odds of ML resistance were increased: on farms that had used long-acting ML products in ewes as a pre-lambing treatment for ≥3 of the previous 5 years (odds ratio (OR) = 7.2; 95% confidence interval (CI) = 1.7–30.3); on farms where <70% of the total stock units mid-winter were from sheep (OR=6.5; 95% CI=1.6–25.6); on farms which over the year purchased ≥10% of the number of sheep present mid-winter (OR=7.1; 95% CI=1.5–34.7); and on farms where the average wool diameter of the main flock was <37 (OR=4.1; 95% CI=1.1–14.7) microns. The model provided a good fit to the data (pseudo R²=0.64; Hosmer-Lemeshow statistic = 0.38).

CONCLUSIONS: Explanatory factors identified as associated with the presence of ML (ivermectin) resistance on farms included the use of long-acting anthelmintic formulations in ewes pre-lambing, sources of refugia of unselected parasites on the farm, breed of sheep and their requirements for anthelmintic treatments, and the importing of resistant parasites with purchased stock. The study provides support for controls that aim to provide refugia of susceptible worms and that minimise the risk of introduction of resistance through effective quarantine-drenching.